Organic disulfides are useful chemicals for pre-sulfiding catalysts and as chemical intermediates in the production of agricultural and pharmaceutical products. Organic disulfides are produced by oxidation of mercaptans according to the general reaction EQU 2R--SH+Oxidant.fwdarw.R--S--S--R+Reductant (1)
The most common oxidants for consideration are sulfur, hydrogen peroxide, a reducible metal ion, and oxygen. Examples of these are as follows: EQU 2R--SH+S.fwdarw.R--S--S--R+H.sub.2 S (2) EQU 2R--SH+H.sub.2 O.sub.2 .fwdarw.R--S--S--R+2H.sub.2 O (3) EQU 2R--SH+2Fe.sup.+3 .fwdarw.R--S--S--R+2Fe.sup.+2 +2H.sup.+ (4) EQU 2Fe.sup.+2 +O.sub.2 +2H.sup.+ .fwdarw.2Fe.sup.+3 +H.sub.2 O(5) EQU 2R--SH+O.sub.2 R--S--S--R+H.sub.2 O (6)
In order to minimize production of polysulfides, when sulfur is used as oxidant, excess mercaptan is used, generally in at least 50-100% excess. The mercaptan then needs to be recovered from the by-product hydrogen sulfide for recycle. Also, the disulfide must be recovered from the polysulfides formed.
The use of hydrogen peroxide suffers from the production of 2 moles of water per mole of disulfide produced as well as the additional water present with the aqueous solution of hydrogen peroxide. Thus, the reactor volume productivity is low for this method.
The use of stoichiometric amount of metal ion such as ferric nitrate is a possibility (reaction 4). However, this process also suffers from low volume productivity of disulfide produced per reactor volume. The advantage is that the ferrous ion can be regenerated with air to produce ferric ion (reaction 5).
The other alternative is to use oxygen as the oxidant. This reaction can achieve high conversions (&gt;99.5%) with high selectivity (&gt;98%). The reactions are done in the presence of a basic catalyst, such as caustic, triethylamine or phase transfer catalysts such as TERGITOL.RTM.. Based on reaction (4), it is anticipated that metal ions can serve as co-catalysts for the oxidation of mercaptans.
The use of oxygen does have a significant potential safety issue due to the potential for explosions under certain conditions. The production of the lower alkyl sulfides such as, for example, dimethyl disulfide poses an additional concern due to the high partial pressure of methyl mercaptan. It has been experimentally shown that, at the pressure of 400 psig and temperature of 50.degree. C. used in the production of dimethyl disulfide, 10 volume % oxygen is the lower explosive limit for methyl mercaptan. At concentrations higher than 10 volume % oxygen, there is a risk of explosion if there is an ignition source also present. For safety considerations, the reaction is performed under conditions where the oxygen concentration is kept to less than about 10 volume %, preferably less than 7 volume %, and most preferably less than 5 volume %. Therefore, there is a need to develop a process for oxidizing a mercaptan to an organic disulfide using oxygen in the safe region and achieving the highest possible yield of disulfide.